1. Technical Field of the Invention
The present invention relates in general to direct current (dc) motor performance testing and, more particularly, to methods and apparatus for determining performance characteristics of a dc motor without contacting the motor shaft.
2. Description of the Related Art
It has long been a practice in the manufacture of electrical motors to test and publish specific performance characteristics for the motor designs so that the motors can be used most effectively and in appropriate applications. Two relationships that provide useful performance characteristics are motor speed versus torque, and current drawn by the motor versus torque.
In conventional testing arrangements, the shaft of a motor to be tested is coupled to a secondary device during measurements of motor characteristics including speed, current draw and torque. Additional devices, such as torque transducers and brakes, are also often used for the testing. Unfortunately, the use of couplings can lead to problems including shaft binding, misalignment and of course the added time and work required to connect the coupling to the motor shaft and to disconnect the coupling from the motor shaft. Such problems with couplers and the devices connected to a motor shaft for testing can affect the test results and can also cause heat build up within the motor, particularly within the armature, and such heat can be sufficient to skew the results of the test. Such problems can not only affect the test results but also can prevent tests from being repeated consistently. Further, the bulk and awkward operation of such testing devices have made it difficult if not impossible to practically test motors in a production environment.
The present inventors have recognized that testing a motor without the use of a coupling device and other secondary components can result in more reliable motor performance measurements, notably stall current, stall torque, vibration measurements and motor noise measurements. Further, the present inventors have recognized that fixtures required for testing can be greatly simplified, cost can be reduced, and test times can be reduced by a testing arrangement that does not require coupling anything to the shaft of a motor to be tested.
Accordingly, there is a need for improved methods and apparatus for determining motor performance characteristics without contacting the shaft of a motor to be tested by using coupling devices, brakes, torque transducers and the like. Preferably, the improved methods and apparatus would tend to reduce the amount of motor heating thus improving test reliability and repeatability, would be economical and would permit practical motor testing in a production environment.
This need is met by the invention of the present application wherein methods and apparatus provide for motor testing of performance characteristics for motors exhibiting linear operating characteristic curves without contacting shafts of the motors during testing. More particularly, a motor to be tested is operated in one direction to a no-load speed and then reversed with current and speed measurements being made during this testing period. A full load or stall point or points and a no-load point or points are determined from the measurements so that linear speed versus torque and/or current versus torque motor characteristic curves can be generated for the motor. Multiple voltages can be applied to determine a family of characteristic curves for the motor.
In accordance with one aspect of the present invention, a method for testing motor performance characteristics includes determining a stall current for the motor. This step can occur at any time throughout this method. The stall current is the current peak determined when the shaft is at zero speed and a voltage is applied to the motor. Traditionally, this is accomplished by loading the shaft down to a stall. By the present invention however, the shaft is never loaded. Depending upon the sensitivity of available testing equipment, the stall current can be determined in either one of two ways. If transient signals can be captured quickly enough, the stall current can be determined by measuring a maximum current drawn when the motor is initially started from rest. However, it is sometimes difficult to synchronize testing apparatus and make this measurement. An alternative to determine the stall current includes operating the motor at a percentage of the motor rated voltage. Various testing requirements and types of motors tested will determine the direction in which the motor is operated, however it should be noted that this method works generally for both forward and reverse directions. Additionally, the percentage of voltage applied can be low, for example, 20-25% is often satisfactory. Preferably, the motor should be allowed to reach a constant speed, then the motor direction reversed. When the direction of the motor is reversed, the current drawn by the motor is monitored to determine a startup current peak. The stall current is then set equal to the measured startup current peak.
Another step that can be performed during any part of the method is measuring no-load characteristics for the motor. This involves operating the motor at a constant rate of speed and measuring the speed of the shaft in RPMs and the current drawn while operating at the constant speed. As with the previous step, the motor can be operated in either direction for this step.
The next step includes operating the motor in a first direction, preferably at the rated voltage. Once again, this step can be performed in either direction. Once the shaft is rotating, preferably at a constant speed, the direction of operation is reversed so that the motor operates in a second direction opposite to the first direction. This step includes measuring a time interval for a speed change from a no-load operating speed in the first direction to zero speed after reversing operation of the motor. This time interval can be determined by switching the motor from the first direction to the second direction and then measuring motor current drawn while the motor shaft decelerates. The motor current is compared to the stall current and the time interval is set to the time span starting when the motor is reversed up to the time when the motor current substantially equals the stall current. Usually, more accurate results are obtained by the steps of identifying all occurrences where the motor current equals the stall current and setting the time interval to the time span starting when the motor is reversed up to the last time when the motor current substantially equals the stall current. By collecting current data over this period of time, current ripple can be monitored with excessive current ripple indicating a faulty motor.
After completing the steps above, the stall current or stall torque is calculated for the motor. The stall torque is calculated by multiplying a motor no-load speed by a moment of inertia and dividing the result by the product of a stall torque constant and the time interval previously determined. The stall torque constant and moment of inertia are known values for a given motor.
Finally, based upon the data collected, linear speed versus torque and linear current versus torque characteristic curves can be determined. Further, the method can be repeated any number of times at varying voltages to create a family of linear speed versus torque and linear current versus torque characteristic curves, i.e., each voltage applied results in the determination of one of the curves of the family.
In accordance with another aspect of the present invention, a device for testing a motor without contacting a shaft thereof to determine performance characteristics for the motor according to the above described method includes a controller to initiate the steps and record the measurements. Preferably, three voltage sources are provided including a forward voltage power supply, a lower reverse voltage power supply and a higher reverse voltage power supply. These supplies are switchably connected to the motor being tested by a power supply switching circuit controlled by the controller to supply proper test voltages for testing a motor in accordance with the present invention. A power takeoff wired to the power supply switching circuit provides a removable connection between the motor being tested and the test device. A speed sensor is positioned near the shaft of the motor to provide rotational speed measurements. The speed sensor provides input to the controller either directly, or through a conversion device such as a digital input output circuit which can be external to the controller or included within the controller. Finally, a current measuring device provides input to the controller either directly, or through a conversion device such as an analog-to-digital converter. The current measurement device is preferably placed near to the power takeoff and may comprise a current sensor shunt resistor placed between the power supply switching circuit and the power take off, one or more current pickup sensors connected to the current sensor shunt resistor, and a conversion device between the current pickup sensor or sensors and the controller.
Accordingly, it is an object of the present invention to determine motor performance characteristics without the use of coupling devices, brakes, torque transducers or the like.
It is another object of the present invention to provide a method and apparatus to determine motor performance characteristics that reduces the amount of armature heating thus improving test result repeatability.
It is a further object of the present invention to provide a method and apparatus to determine motor performance characteristics that are economical and provide time savings over coupled systems, and that produce reliable, repeatable results.
Other objects and advantages of the present invention will be apparent in light of the description of the invention embodied herein.